1. Field of the Invention:
The present invention relates to a steering system for use in a motor vehicle.
2. Description of the Relevant Art:
Conventional steering systems for motor vehicles or the like include mechanical components such as gear, links, and the like which operatively interconnect a steering wheel and steerable road wheels. Steering action of the steering wheel is transmitted via the mechanical components to the steerable road wheels to turn the road wheels through an angle dependent on a steering angle .theta.s of the steering wheel. In such a steering system, the steering angle .theta.s represents a desired amount (or a desired angle) by which the motor vehicle is to be turned from a reference direction of the motor vehicle (i.e., a straight running direction of the motor vehicle). Therefore, where the steering angle .theta.s is zero, the motor vehicle runs along a straight course without changing its direction of travel.
When a motor vehicle 200 is to change lanes as shown in FIG. 6A of the accompanying drawings, the steering angle .theta.s varies as shown in FIG. 6B while the resultant direction R2 of travel is the same as the initial direction R1 of travel. More specifically, when the motor vehicle 200 starts changing lanes, the steering wheel thereof (not shown) is turned in one direction (to the right in FIG. 6B) from a neutral position (.theta.s=0). When the motor vehicle 200 goes beyond a lane separating line L, the steering wheel is turned in the opposite direction (to the left) beyond the neutral position. When the motor vehicle 200 enters the desired other lane, the steering wheel is turned again in said one direction to allow the motor vehicle 200 to run straight, and then is returned to the neutral position.
When the motor vehicle 200 makes a 90.degree.-turn along a road bend around a corner as shown in FIG. 7A, the resultant direction R3 of travel of the motor vehicle 200 is angularly shifted 90.degree. clockwise from the original direction R1 of travel. At this time, the steering angle .theta.s varies as shown in FIG. 7B. At the start of making the turn, the steering wheel is turned in one direction (to the right in FIG. 7B) from the neutral position. When the motor vehicle 200 completes the 90.degree.-turn, the steering wheel is turned in the opposite direction (to the left) back to the neutral position.
For steering the motor vehicle equipped with the above steering system, the driver may apply, as one steering input, a steering angle .theta.s as a basis for producing a turning angle .delta. of a steerable road wheel (e.g., a front wheel turning angle .delta.f), to the steering system. The behavior of the motor vehicle while it is making a turn is greatly affected by lateral motion (transverse motion) and yawing. While the motor vehicle is making a stable turn, the lateral acceleration y of the motor vehicle, a yaw rate (yaw angular velocity) r, and the radius R of the turning circle are related to the turning angle .delta. (here, the front wheel turning angle .delta.f) and the vehicle speed V as follows: ##EQU1## where l: the wheelbase, and
Kf: the stability factor indicative of the response of the motor vehicle.
As can be understood from the above equations (1), (2), and (3), in order to get a desired turning behavior of the motor vehicle, the driver is required to achieve an appropriate turning angle .delta.f by giving a steering angle (.theta.s) in view of the vehicle speed V.
When the speed of movement of the motor vehicle is high, and the lateral slip angle of the tires is large, the cornering forces produced by the tires become nonlinear. In such a case, the stability factor Kf is also varied as it is affected by the transient characteristics. Therefore, the driver is also required to turn the steering wheel while taking into account such a change in the stability factor Kf.